Support custom filename to be provided to URLFile (#2004)
This commit works around an issue where the basename of the URL many not
actually contain a file extension and the uploader logic cannot infer
the mime type for the file.
We stash the name when pickling and extract it again when unpickling.
The __getattr__ function then supports returning the underlying name
value rather than proxying to the underlying request object.
I also ran into a small bug whereby the __del__ method was triggering
a network request because of some private attributes being accessed
during teardown would trigger the __wrapper__ code. I've overridden
the super class to disable this. Though I'm unclear if this is just the
test suite doing this cleanup.
